DESCRIPTION

The three functions
hcreate(),
hsearch(),
and
hdestroy()
allow the caller to create and manage a hash search table
containing entries consisting of a key (a string) and associated data.
Using these functions, only one hash table can be used at a time.

The three functions
hcreate_r(),
hsearch_r(),
hdestroy_r()
are reentrant versions that allow a program to use
more than one hash search table at the same time.
The last argument,
htab,
points to a structure that describes the table
on which the function is to operate.
The programmer should treat this structure as opaque
(i.e., do not attempt to directly access or modify
the fields in this structure).

First a hash table must be created using
hcreate().
The argument nel specifies the maximum number of entries
in the table.
(This maximum cannot be changed later, so choose it wisely.)
The implementation may adjust this value upward to improve the
performance of the resulting hash table.

The
hcreate_r()
function performs the same task as
hcreate(),
but for the table described by the structure
*htab.
The structure pointed to by
htab
must be zeroed before the first call to
hcreate_r().

The function
hdestroy()
frees the memory occupied by the hash table that was created by
hcreate().
After calling
hdestroy()
a new hash table can be created using
hcreate().
The
hdestroy_r()
function performs the analogous task for a hash table described by
*htab,
which was previously created using
hcreate_r().

The
hsearch()
function searches the hash table for an
item with the same key as item (where "the same" is determined using
strcmp(3)),
and if successful returns a pointer to it.

The argument item is of type ENTRY, which is defined in
<search.h> as follows:

typedef struct entry {
char *key;
void *data;
} ENTRY;

The field key points to a null-terminated string which is the
search key.
The field data points to data that is associated with that key.

The argument action determines what
hsearch()
does after an unsuccessful search.
This argument must either have the value
ENTER,
meaning insert a copy of
item
(and return a pointer to the new hash table entry as the function result),
or the value
FIND,
meaning that NULL should be returned.
(If
action
is
FIND,
then
data
is ignored.)

The
hsearch_r()
function is like
hsearch()
but operates on the hash table described by
*htab.
The
hsearch_r()
function differs from
hsearch()
in that a pointer to the found item is returned in
*retval,
rather than as the function result.

RETURN VALUE

hcreate()
and
hcreate_r()
return nonzero on success.
They return 0 on error.

On success,
hsearch()
returns a pointer to an entry in the hash table.
hsearch()
returns NULL on error, that is,
if action is ENTER and
the hash table is full, or action is FIND and item
cannot be found in the hash table.
hsearch_r()
returns nonzero on success, and 0 on error.

ERRORS

hcreate()
and
hcreate_r()
can fail for the following reasons:

EINVAL

(hcreate_r())
htab
is NULL.

ENOMEM

Table full with action set to ENTER.

ESRCH

The action argument is FIND and no corresponding element
is found in the table.

hsearch()
and
hsearch_r()
can fail for the following reasons:

ENOMEM

action
was
ENTER,
key
was not found in the table,
and there was no room in the table to add a new entry.

ESRCH

action
was
FIND,
and
key
was not found in the table.

POSIX.1-2001 only specifies the
ENOMEM
error.

CONFORMING TO

The functions
hcreate(),
hsearch(),
and
hdestroy()
are from SVr4, and are described in POSIX.1-2001.
The functions
hcreate_r(),
hsearch_r(),
and
hdestroy_r()
are GNU extensions.

NOTES

Hash table implementations are usually more efficient when the
table contains enough free space to minimize collisions.
Typically, this means that
nel
should be at least 25% larger than the maximum number of elements
that the caller expects to store in the table.

The
hdestroy()
and
hdestroy_r()
functions do not free the buffers pointed to by the
key
and
data
elements of the hash table entries.
(It can't do this because it doesn't know
whether these buffers were allocated dynamically.)
If these buffers need to be freed (perhaps because the program
is repeatedly creating and destroying hash tables,
rather than creating a single table whose lifetime
matches that of the program),
then the program must maintain bookkeeping data structures that
allow it to free them.

BUGS

SVr4 and POSIX.1-2001 specify that action
is significant only for unsuccessful searches, so that an ENTER
should not do anything for a successful search.
In libc and glibc (before version 2.3), the
implementation violates the specification,
updating the data for the given key in this case.

Individual hash table entries can be added, but not deleted.

EXAMPLE

The following program inserts 24 items into a hash table, then prints
some of them.